Being tabbed and actually being in a Ranger Batt are two totally separate entities. Tabbed means you are "Ranger" qualified and is essentially required by most infantry officers if you want a leg up in promotion. It is basically mandated that if you branch infantry you will at least attempt Ranger school.
As a member of the Ranger Regiment it is considerably more difficult and they have there own ways of assessing officers candidates (and they are of course all tabbed).
Aglahad summed it up nicely.
For the original poster, consider this;
First you will need to be accepted and enroll in college. Don't count on a scholarship, you may or may not receive one, it will be very competitive this year. From your previous posts you first interest was with the Marines, I would imagine you will be applying to both for a scholarship. If you do not receive one you will need to decide which ROTC you wish to join.
If you choose Army and attend with a scholarship you will then need to keep your grades at a minimum to stay in the program and keep your scholarship, even if you do not have a scholarship you will be required to meet GPA and ROTC requirements to be able to move into the Advanced program your junior year and become contracted.
During this time you will be competing with all the cadets in country in areas such as GPA, PT and later LDAC as well as your battalion OML. This competition will come to a conclusion with you being ranked on the national OML.
That brings me to the next hurdle, you will also be competing to commission Active Duty, not everyone gets Active Duty, fall to low on the OML and you will be forced Reserves and that cutoff line will only be going higher in the coming years.
Now for the next hurdle, you will now be competing for your branch. Infantry is one of the hardest branches to get, if your not near the top don't count on it, and don't kid yourself, it takes a lot of hard work and a little luck to be at the top. GPA will be one of the biggest factors you can control, next is your APFT scores, again in your control, LDAC, well, that's where some luck comes into play as well as your skill level.
So now you ready to commission, if all the planets have aligned and you are high enough on the list to branch Infantry, the work is just beginning. You will now need to pass Infantry School, then get good fitness reports as a Platoon Leader, then it's off to Ranger School, again not an easy school to pass. Now become Captain promotable and you may, just may get the opportunity to apply and enter the 75th Rangers. You will need to be Captain promotable to apply for SF as well, one thing to remember, you do not need to be an Infantry Officer to apply for SF. I believe you can be a Officer in any branch and still apply for SF, this again is not an easy thing to do.
As Aglahad said, once you complete Ranger School you are only Ranger Qualified, it does not make you a Ranger. Make sure you keep that distinction clear in your mind, refer to yourself as a Ranger and there will be many who will correct you quickly and not kindly.
My main point here is that you have a lot of hoops to jump through before you get to that brass ring. Take them one at a time, you my find that you completely change directions once you start, many do.
PIMA said it best, your goal right now should be becoming an officer in the US Military, whichever branch you choose. You are an Officer first and always, your branch is just what you do for a living.
Just a question, have you applied for scholarships for any of the ROTC programs. If you have I hope you didn't use the "I just want to kill terrorists" line in your interview, you might want to tone that down a bit.
Nothing is a guarantee when it comes to ROTC and the Military, you will need to be able to adapt to what ever comes your way.